Flow cytometry, discontinuous-gradient cell isolation, amino acid analysis, dissociated cell culture, immunoblots, cell migration, immunochemistry, in situ hybridization and PCR methods are applied to embryonic-early postnatal rat CNS tissues to study the development, differentiation and cellular distribution of transmitter, transmitter-related enzymes and their corresponding receptors. During the past several years, we have focussed intensely on GABA, which is transiently expressed in a virtually ubiquitous manner during CNS development before it becomes restricted to fast inhibitory synapses in the adult. In FY 93 we investigated the following: 1) transcripts encoding two GABA-synthesizing enzymes emerge at E13 in the thalamus and are expressed by virtually every cell during development; 2) transcripts encoding 8 GABA receptor subunit proteins emerge at E14 in the thalamus, are expressed until the second week postnatal when all but two become undetectable with these latter two being joined by two new transcripts to complete the adult GABA receptor form; 3) PCR reveals transcripts encoding 9 GABA receptor subunit proteins at E12 in the spinal cord when only one subunit can be detected by in situ; 4) in situ reveals three distinctive patterns of transcript coexpression in the cord: one exclusively in the neuroepithelial proliferative zone, one in most cells during the embryonic-postnatal period and one emerging during postnatal differentiation; 5) by densitometry of in situ signals and PCR of the 4 transcripts that remain in the adult, it is clear that all 4 are several-fold more abundant during development; 6) the chemokinetic effects of GABA on embryonic spinal cord cells are dose-dependent, with cells migrating to fM and pM concentrations, and age- and region~dependent, with ventral cells migrating before dorsal cells; 7) chemokinetic effects of GABA can be mimicked by both baclofen and muscimol, suggesting novel GABA receptor structure-activity relations; 8) pertussis toxin inhibits GABA-induced chemokinesis, but not chemotaxis induced by NGF, implicating G protein-mediated signal transduction in these novel effects of GABA's; 9) aM levels of GABA, muscimol and baclofen all release cytoplasmic free Ca2+ during the embryonic period of cortical development; 10) embryonic cells differentiating in culture can be organized into specific patterns using defined surfaces and substrates.